This invention is related to the production of arylhydroxylamines from nitroaromatic compounds. More particularly, it is directed to methods for rapidly producing such arylhydroxylamines in high yields without producing large quantities of coproducts, such as aniline.
Arylhydroxylamines are precursors in the synthesis of diarylnitrones. Certain diarylnitrones have been found to be useful in contrast enhancement photolithography. Methods for the production of these diarylnitrones and their use are more particularly described in copending applications Ser. Nos. 735,016, 675,915, 676,918 and 687,681, all assigned to the same assignee as the present invention. The contents of these copending applications are incorporated herein by reference. The arylhydroxylamines provide arylnitrones typically by reaction with an aldehyde, preferably arylaldehydes.
These arylhydroxylamines are prepared by the reduction of nitroaromatic compounds. Reduction of nitroaromatics has been accomplished by metal reductions, electrochemical reductions and catalytic hydrogenation reactions. Electrochemical reductions are very costly and therefore, are not preferred. Metal reductions are effective but pose problems in waste disposal and in overreducing the nitroaromatics to aromatic amines. Catalytic hydrogenation is a very attractive scheme, however, over reduction to aromatic amines is also a problem. Rylander et al., U.S. Pat. No. 3,694,509, discloses a catalytic hydrogenation reaction which is moderated to prevent overreduction of the nitroaromatics. Rylander et al. utilizes a precious metal catalyst in neutral media and the moderator dimethylsulfoxide (DMSO) to produce N-arylhydroxylamines. Other moderated reduction reactions have been disclosed such as the divalent sulphur compounds of Caskey et al. U.S. Pat. No. 4,415,753; tetravalent sulfoxides, U.S. Pat. No. 3,897,499; organic bases, U.S. Pat. Nos. 3,927,101 and 3,992,395 and the phosphorus compounds disclosed in Japanese Pat. No. 54[1979]-24837. Although these moderated reactions are selective in producing arylhydroxylamines, there remains room for improvement. Selectivity is obtained in these reactions by depressing the rate of reduction. As a result, these reactions are relatively slow when compared to unmoderated reactions. It is desirable to enhance the rate of reduction in these reactions without losing selectivity.
The addition of acid to an unmoderated reduction reaction has been shown to have no catalytic effect by Makaryan et al. in Investiya Akademii Nauk SSSR, No. 4, pp. 750-764, (April, 1983) as translated by the Academy of Sciences of the USSR, 692-695 (1983). Rylander (U.S. Pat. No. 3,715,397) has shown that the reduction of nitroaromatics in aqueous solutions of 25% sulfuric acid, in the presence of the moderator DMSO, results in aminophenol rearrangement products.
The principal object of the present invention, therefore, is to provide moderated catalytic hydrogenation reactions wherein the nitroaromatic compounds are not overreduced. Another object of the present invention is to enhance the rate of reduction of nitroaromatics and increase the yield of arylhydroxylamines from moderated reactions without a loss in selectivity and without producing rearrangement products. Other objects will be obvious from the discussions herein.